Power converter

ABSTRACT

A power converter can include an inverter including, connected in series, switching elements, the inverter circuit being connected to both ends of a DC power source series circuit resulting from connecting in series a DC power source and a DC power source. Also included can be an AC output terminal that is connected to a connection point of the switching element and the switching element, an AC output terminal that is connected to a connection point of the DC power source and the DC power source, a bidirectional switch element one end of which is connected to the AC output terminal and the other end of which is connected to a terminal of an AC power source and a bidirectional switch element one end of which is connected to the AC output terminal and the other end of which is connected to the AC power source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2012/004394, filed on Jul. 6, 2012, which is based on and claimspriority to Japanese Patent Application No. JP 2012-053322, filed onMar. 9, 2012. The disclosure of the Japanese priority application andthe PCT application in their entirety, including the drawings, claims,and the specification thereof, are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a power converter that generatespredefined AC voltage using voltage of an AC power source and a DC powersource. More particularly, the present invention relates toinstantaneous voltage-drop compensation devices and an uninterruptiblepower source device that can supply stabilized voltage to a load, evenupon occurrence of fluctuation of voltage of an AC power source andpower failure in the AC power source.

2. Related Art

FIG. 15 is a diagram for explaining a power converter of acontinuous-inverter power feeding scheme disclosed in Japanese PatentApplication Publication No. H7-337036 (also referred to herein as“Patent document 1”). The power converter converts temporarily thevoltage of an AC power source to DC voltage, converts the DC voltageagain to AC voltage, and supplies the voltage to a load. In the figure,the reference symbol 1 is a single-phase AC power source, 2 is acapacitor, 3 is a converter circuit, 4 is an inverter circuit, 5 is afilter circuit, and 6 is a load.

One end of the AC power source 1 is connected to an AC input terminal ofthe converter circuit 3. The AC input terminal of the converter circuit3 is connected to one end of the AC power source 1. In the convertercircuit 3, one end of a reactor L is connected to the AC input terminal.The other end of the reactor L is connected to a connection midpoint ofa switching element series circuit, in which switching elements Qp, Qnare connected in series. A capacitor series circuit in which capacitorsCp, Cn are connected in series, is connected to both ends of theswitching element series circuit. The connection midpoint of thecapacitor series circuit is connected to the other end of the AC powersource 1. The converter circuit 3 turns the switching elements Qp, Qn onand off, to rectify the voltage of the AC power source 1, and charge thecapacitors Cp, Cn to a predefined voltage. The capacitors Cp, Cn thuscharged to a predefined voltage form a DC power source.

The capacitor 2 is connected between the AC input terminal of theconverter circuit 3 and the connection midpoint of the capacitor seriescircuit. The inverter circuit 4 comprises series-connected switchingelements Q1, Q2. The inverter circuit 4 is connected to a DC outputterminal of the converter circuit 3. The inverter circuit 4 turns on andoff the switching elements Q1, Q2, to convert, to AC voltage, thevoltage of the DC power source that comprises the capacitors Cp, Cn.

The filter circuit 5 is configured through connection in series of areactor Lf1 and a capacitor Cf1. One end of the filter circuit 5 isconnected to the connection midpoint of the switching elements Q1, Q2.The other end of the filter circuit 5 is connected to the connectionmidpoint of the capacitor series circuit. The filter circuit 5 removes ahigh-frequency component from the AC voltage that is outputted from theinverter circuit 4.

One end of the load 6 is connected to the connection point of thereactor Lf1 and the capacitor Cf1. The other end of the load 6 isconnected to the other end of the AC power source 1. The AC voltage thatis supplied from the inverter circuit 4 is outputted, via the filtercircuit 5, to the load 6.

FIG. 16 is a diagram for explaining a power converter of a continuouscommercial-power feeding scheme disclosed in Japanese Patent ApplicationPublication No. H11-178216 (also referred to herein as “Patent document2”). In the figure, a switch 7 and the secondary winding of atransformer 8 are connected in series between an AC power source 1 and aload. The respective connection relationships between a convertercircuit 3, an inverter circuit 4, a filter circuit 5 and a capacitor 2are identical to those of the embodiment in FIG. 15. An AC inputterminal of the converter circuit 3 is connected to one end of theprimary winding of the transformer 8. The connection midpoint of thecapacitor series circuit is connected to the other end of the AC powersource 1 and is connected to the other end of the primary winding of thetransformer 8. The connection point of the reactor Lf1 and the capacitorCf1 is connected to one end of the load 6.

The power converter supplies ordinarily voltage of the AC power sourceto the load. When the voltage of the AC power source 1 drops, theconverter circuit 3 turns on and off the switching elements Qp, Qn, togenerate thereby compensation voltage for compensating the voltage dropfrom the DC voltage at which the capacitor series circuit is charged.

The compensation voltage is superimposed on the voltage of the AC powersource 1, via the transformer 8. The voltage resulting fromsuperimposing the compensation voltage on the voltage of the AC powersource 1 is supplied to the load 6. Charging of the capacitor seriescircuit is carried out in this case by the inverter circuit 4.

The switch 7 is opened when the AC power source 1 fails. The invertercircuit 4 turns on and off the switching elements Q1, Q2, to convert theDC voltage of the capacitor series circuit to AC voltage, and supply thevoltage to the load 6.

In the power converter illustrated in FIG. 15, however, AC-DC voltageconversion by the converter circuit 3 and DC-AC voltage conversion bythe inverter circuit 4 are required until AC voltage is supplied fromthe AC power source 1 to the load 6. The current that flows through thepower converter passes at least once through each switching element ofthe converter circuit 3 and the inverter circuit 4. That is, the currentflowing in the power converter passes through switching elements atleast two or more times. Accordingly, respective conduction lossesderived from passage of current through the switching elements occur atboth the converter circuit 3 and of the inverter circuit 4.

The on and off operations of the switching elements Q1 to Q4 in theconverter circuit 3 and the inverter circuit 4 are performed on thebasis of the voltage of the DC power source that comprises thecapacitors Cp and Cn. Accordingly, switching loss occurs when eachelement is turned on or turned off.

The power loss, including conduction loss and switching loss, in theswitching elements is therefore substantial in the technology disclosedin Patent document 1. A problem arises herein in that the conversionefficiency of the power converter drops when power loss in the switchingelements is large.

In the power converter illustrated in FIG. 16, the transformer 8 isrequired in order to compensate for the voltage drop of the AC powersource 1. The size of the transformer 8 is large, since the latter mustfunction at a commercial frequency. In the power converter illustratedin FIG. 16, moreover, the operations of the converter circuit 3 and theinverter circuit 4 must be switched in order to supply predefined ACvoltage to the load 6 when a power failure occurs in the AC power source1.

A problem arises therefore, in the technology disclosed in Patentdocument 2, in that a large commercial transformer is required, whichtranslates into a power converter of large size. A further problem isthe occurrence of disturbances in the AC output voltage as a result ofswitching over between the operations of the converter circuit 3 and theinverter circuit 4.

SUMMARY OF THE INVENTION

A goal of the present invention is to solve problems such as those ofconventional technologies. Specifically, it is an object of the presentinvention to provide a power converter that can output AC outputvoltage, without occurrence of disturbances, during a voltage drop of anAC power source or during power failure in the AC power source. It is afurther object of the present invention to provide a power converterthat allows reducing power loss. Yet a further object of the presentinvention is to provide a power converter that requires nocommercial-frequency transformer to perform a voltage compensationoperation.

In order to attain the above goal, a first embodiment in which thepresent invention is used is a power converter that outputs AC voltageon the basis of an AC voltage command, comprising: a single-phase ACpower source having a first AC terminal and a second AC terminal; and aDC power source series circuit, resulting from connecting in series afirst DC power source and a second DC power source such that a neutralterminal, which is a connection point of the first DC power source andthe second DC power source, is connected to the second AC terminal. Thepower converter outputs voltage of any one level selected from amongfour levels of voltage of a positive-side voltage of the DC power sourceseries circuit, a negative-side voltage of the DC power source seriescircuit, zero voltage being the potential of the neutral terminal, and avoltage of the AC power source.

In the power converter, further, the positive-side voltage of the DCpower source series circuit and the negative-side voltage of the DCpower source series circuit are larger than an amplitude value of thevoltage of the AC power source.

In the power converter, further, at each control period that iscontinuous and has a pre-established time width, the power convertersets, as a first voltage, a voltage of one level selected from among thefour levels of voltage, and sets, as a second voltage, a voltage ofanother level selected from among the four levels of voltage. The powerconverter outputs complementarily the first voltage and the secondvoltage over respective predefined time widths.

In the power converter, further, the first voltage and the secondvoltage are voltages selected on the basis of the AC output voltagecommand and the voltage of the AC power source.

In the power converter, further, at each control period that iscontinuous and has a pre-established time width, the power converterselects as a first voltage, from among the four levels of voltage, avoltage having an absolute value equal to or higher than an absolutevalue of the AC output voltage command and having a value that isclosest to a value of the AC output voltage command, and selects as asecond voltage, from among the four levels of voltage, a voltage havingan absolute value smaller than the absolute value of the AC outputvoltage command, and having a value that is closest to the value of theAC output voltage command. The power converter sets a time ratio of anoutput time of the first voltage and an output time of the secondvoltage, within each control period, to a predefined value, andcomplementarily outputs the first voltage and the second voltage, tothereby output AC voltage corresponding to the AC output voltagecommand.

At each control period that is continuous and has a pre-established timewidth, the power converter sets the positive-side voltage of the DCpower source series circuit as the first voltage and sets the zerovoltage as the second voltage when the AC output voltage command isequal to or higher than the zero voltage and the voltage of the AC powersource is lower than the zero voltage. The power converter sets thepositive-side voltage of the DC power source series circuit as the firstvoltage and sets the voltage of the AC power source as the secondvoltage when the AC output voltage command is equal to or higher thanthe zero voltage, the voltage of the AC power source is equal to orhigher than the zero voltage, and the AC output voltage command ishigher than the voltage of the AC power source. The power converter setsthe voltage of the AC power source as the first voltage and sets thezero voltage as the second voltage when the AC output voltage command isequal to or higher than the zero voltage, the voltage of the AC powersource is equal to or higher than the zero voltage and the AC outputvoltage command is equal to or lower than the voltage of the AC powersource. The power converter sets the voltage of the AC power source asthe first voltage and sets the zero voltage as the second voltage whenthe AC output voltage command is lower than the zero voltage, thevoltage of the AC power source is lower than the zero voltage and the ACoutput voltage command is equal to or higher than the voltage of the ACpower source. The power converter sets the negative-side voltage of theDC power source series circuit as the first voltage and sets the voltageof the AC power source as the second voltage when the AC output voltagecommand is lower than the zero voltage, the voltage of the AC powersource is lower than the zero voltage and the AC output voltage commandis lower than the voltage of the AC power source. The power convertersets the negative-side voltage of the DC power source series circuit asthe first voltage and sets the zero voltage as the second voltage whenthe AC output voltage command is lower than the zero voltage and thevoltage of the AC power source is equal to or higher than the zerovoltage. The power converter sets a time ratio of an output time of thefirst voltage and an output time of the second voltage, within eachcontrol period, to a predefined value, and complementarily outputs thefirst voltage and the second voltage, to thereby output AC voltagecorresponding to the AC output voltage command.

In the power converter, the output time of the first voltage isestablished on the basis of the AC output voltage command, the firstvoltage and the second voltage. In the power converter, the output timeof the second voltage is a time resulting from subtracting the outputtime of the first voltage from the time of each control period.

In the power converter, further, the output time of the first voltage isa time corresponding to a value obtained by dividing a differencevoltage between the AC output voltage and the second voltage by adifference voltage between the first voltage and the second voltage.

In the power converter, further, an average value of the AC outputvoltage in each control period is equal to an average value of the ACoutput voltage command within that control period.

In the power converter, further, the AC output voltage command issynchronized with the voltage of the AC power source. In the powerconverter, further, the power converter outputs the voltage of the ACpower source when a deviation between the voltage of the AC power sourceand the AC output voltage command lies within a pre-established range.

Further, the power conversion circuit that outputs single-phase ACvoltage in a power converter according to a first embodiment isconfigured out of an inverter circuit and a bidirectional switchcircuit. The inverter circuit is a switching element series circuitresulting from connecting in series a positive-side switching elementthat is connected to a positive-side terminal of the DC power sourceseries circuit, and a negative-side switching element that is connectedto a negative-side terminal of the DC power source series circuit. Thebidirectional switch circuit comprises a first bidirectional switchelement one end of which is connected to the first AC output terminal,which is connected to the positive-side switching element and thenegative-side switching element, and the other end of the firstbidirectional switch element being connected to one end of the AC powersource; and a second bidirectional switch element one end of which isconnected to the first AC output terminal, and the other end of which isconnected to the neutral terminal of the DC power source series circuit.

The power converter in which the present invention is used allowsoutputting voltage of any one level selected from among four levels ofvoltages, namely the positive-side voltage of the DC power source seriescircuit, the negative-side voltage of the DC power source seriescircuit, a neutral voltage of the DC power source series circuit, andthe voltage of the AC power source. Herein, current need only passthrough one bidirectional switch element during output of the voltage ofthe AC power source.

The power converter in which the present invention is used allowsreducing the voltage that is applied to switching elements andbidirectional switch elements. Therefore, the power converter in whichthe present invention is used allows reducing power loss generated inswitching elements and bidirectional switch elements.

The power converter in which the present invention is used divides aperiod of AC output voltage command into a plurality of control periods,and in each control period, outputs, over a respective predefined time,a first voltage, from among the four levels of voltage, being a voltagehaving an absolute value equal to or higher than an absolute value ofthe AC output voltage command and having a value that is closest to theAC output voltage command, and a second voltage, from among the fourlevels of voltage, being a voltage having an absolute value smaller thanthe absolute value of the AC output voltage command and having a valuethat is closest to the AC output voltage command, so that a desired ACvoltage can be generated as a result.

Therefore, the power converter in which the present invention is usedallows outputting AC voltage in which fluctuations of a power sourcevoltage are compensated, even when no commercial-frequency transformeris utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a first embodiment of the presentinvention;

FIGS. 2A-2D are diagrams for explaining an embodiment of a bidirectionalswitch element;

FIG. 3 is a block diagram for explaining the operation of a controlcircuit;

FIG. 4 is a diagram for explaining the relationship between domaindetermination, a pulse width command a, and element selection;

FIGS. 5A-5E are diagrams for explaining AC output voltage in domain 1;

FIGS. 6A-6E are diagrams for explaining AC output voltage in domain 2;

FIGS. 7A-7E are diagrams for explaining AC output voltage in domain 3;

FIGS. 8A-8E are diagrams for explaining AC output voltage in domain 4;

FIGS. 9A-9E are diagrams for explaining AC output voltage in domain 5;

FIGS. 10A-10E are diagrams for explaining AC output voltage in domain 6;

FIG. 11 is a diagram for explaining another relationship between domaindetermination, a pulse width command a, and element selection;

FIGS. 12A-12E are diagrams for explaining AC output voltage in domain 7;

FIG. 13 is a diagram for explaining a second embodiment of the presentinvention;

FIG. 14 is a diagram for explaining a third embodiment of the presentinvention;

FIG. 15 is a diagram for explaining a conventional power converter; and

FIG. 16 is a diagram for explaining a conventional power converter.

DETAILED DESCRIPTION

Embodiments of the power converter of the present invention areexplained in detail next with reference to FIG. 1 to FIG. 14. The powerconverter illustrated in FIG. 1 to FIG. 14 can be used in devices forsupplying stable voltage to a load, even upon occurrence of voltagefluctuation of an AC power source or power failure of the AC powersource, for instance in instantaneous voltage-drop compensation devices,uninterruptible power source devices and the like.

FIG. 1 is a diagram for explaining a first embodiment of the powerconverter of the present invention. The power converter according tothis embodiment generates four levels of voltage using the voltage of anAC power source and the voltage of a DC power source series circuitresulting from connecting two DC power sources in series.

In the figure, the reference symbol 1 is an AC power source, thereference symbol 2 is a capacitor, the reference symbol 30 is a DC powersource series circuit, the reference symbol 4 is an inverter circuit,the reference symbol 5 is a filter circuit, the reference symbol 6 is aload, the reference symbol 100 is a bidirectional switch circuit, andthe reference symbol 200 is a control circuit.

The AC power source 1 is a single-phase AC power source having aterminal R and a terminal S. A capacitor 2 is connected between theterminal R and the terminal S of the AC power source 1. The DC powersource series circuit 30 is a DC power source resulting from connectinga DC power source Psp and a DC power source Psn in series. The DC powersource Psp is a positive-side DC power source. One end of the DC powersource Psp is a positive-side terminal P that outputs voltage ofpositive polarity. The DC power source Psn is a negative-side DC powersource. One end of the DC power source Psn is a negative-side terminal Nthat outputs voltage of negative polarity. A series connection point ofthe DC power source Psp and the DC power source Psn is a neutralterminal O that outputs zero voltage. The neutral terminal O isconnected to the terminal S of the AC power source 1.

The inverter circuit 4 is connected between the positive-side terminal Pand the negative-side terminal N of the DC power source series circuit30. The inverter circuit 4 is configured in the form of a switchingelement series circuit. The switching element series circuit is acircuit resulting from connecting in series a switching element Q1 oneend of which is connected to the positive-side terminal P of the DCpower source series circuit 30, and a switching element Q2 one end ofwhich is connected to the negative-side terminal N.

The series connection point of the switching element Q1 and theswitching element Q2 is connected to an AC output terminal U (first ACoutput terminal) for outputting single-phase AC voltage from theinverter circuit 4. The neutral terminal O of the DC power source seriescircuit 30 is connected to an AC output terminal V (second AC outputterminal) for outputting single-phase AC voltage from the invertercircuit 4.

The bidirectional switch circuit 100 comprises a bidirectional switchelement S1 (first bidirectional switch element) and a bidirectionalswitch element S2 (second bidirectional switch element). One end of thebidirectional switch element S1 is connected to the AC output terminalU, and the other end is connected to the terminal R of the AC powersource 1. One end of the bidirectional switch element S2 is connected tothe AC output terminal U, and the other end is connected to the neutralterminal O of the DC power source series circuit 30.

The AC output terminals U, V are connected to the load 6 via the filtercircuit 5. The filter circuit 5 removes the harmonic component in the ACvoltage that is outputted from the inverter circuit 4. FIG. 2A to FIG.2D illustrate configuration examples of the bidirectional switchelements S1, S2. The bidirectional switch element illustrated in FIG. 2Ais configured through anti-parallel connection of two reverseblocking-type IGBTs (Insulated Gate Bipolar Transistors). Thebidirectional switch element illustrated in FIG. 2B is configuredthrough anti-parallel connection of two sets of switch elements in whichdiodes and IGBTs having no reverse-blocking breakdown voltage areconnected in series. The bidirectional switch element illustrated inFIG. 2C is configured through anti-series connection of two sets ofswitch elements in which diodes are anti-parallel connected to IGBTshaving no reverse-blocking breakdown voltage. The bidirectional switchelement illustrated in FIG. 2D is configured by replacing the IGBTs inthe bidirectional switch element illustrated in FIG. 2C by MOSFETs(Metal Oxide Semiconductor Field Effect Transistors).

The inverter circuit 4 and the bidirectional switch circuit 100 make upa power conversion circuit for generating AC voltage that is supplied tothe load 6. The power conversion circuit operates in such a manner thatany one element from among the switching elements Q1, Q2 and thebidirectional switch elements S1, S2 is selected and turned on, and theother three elements are turned off.

When the switching element Q1 is on, positive voltage of the DC powersource Psp is outputted to the AC output terminal U. When the switchingelement Q2 is turned on, negative voltage of the DC power source Psn isoutputted to the AC output terminal U. When the bidirectional switchelement S1 is turned on, R terminal voltage of the AC power source 1 isoutputted to the AC output terminal U. When the bidirectional switchelement S2 is turned on, zero voltage is outputted to the AC outputterminal U. That is, the power conversion circuit can output, to the ACoutput terminal U, voltage of any one level from among four levels ofvoltage, namely the positive voltage of the DC power source Psp, thenegative voltage of the DC power source Psn, the R terminal voltage ofthe AC power source 1 and the zero voltage, through selection andturning-on of any one element from among the switching elements Q1, Q2and the bidirectional switch elements S1, S2.

The filter circuit 5 is configured through connection in series of areactor Lf1 and a capacitor Cf1. The filter circuit 5 is connectedbetween the AC output terminal U and the AC output terminal V(hereafter, between the AC output terminals U and V). The load 6 isconnected to both ends of the capacitor Cf1. The filter circuit 5removes a harmonic component from the AC output voltage Vus that isoutputted between the AC output terminals U and V. The voltage outputtedfrom the filter circuit 5 is supplied to the load 6.

A control circuit 200 divides the period of a below-described AC outputvoltage command into a plurality of control periods. At each controlperiod, the control circuit 200 generates control signals G1, G2 forturning on and off the switching elements Q1, Q2, and control signalsGs1, Gs2 for turning on and off the bidirectional switch elements S1,S2. Each control period is a continuous period having a pre-establishedtime width.

The length of the control period for controlling the turning on and offof the switching elements Q1, Q2 and the bidirectional switch elementsS1, S2 will be referred to hereafter as a switching period T. Theswitching frequency determined by the switching period T is preferably asufficiently high frequency with respect to the frequency of the ACoutput voltage command. For instance, the switching frequency ispreferably 1 kHz or higher in a case where the frequency of the ACoutput voltage command is a commercial frequency. The switching period Tneed not necessarily be synchronized with one period of the AC outputvoltage command, and may be asynchronous therewith.

FIG. 3 is a block diagram for explaining the operation whereby thecontrol circuit 200 generates control signals. The control circuit 200receives a voltage Vrs of the AC power source 1 as detected by a voltagedetector 301, a voltage Vps of the DC power source Psp as detected by avoltage detector 302, and a voltage Vns of the DC power source Psn asdetected by a voltage detector 303. On the basis of relationshipsbetween these three voltages, the control circuit 200 generates thecontrol signals G1, G2 for controlling the turning on and off of theswitching elements Q1, Q2, and the control signals Gs1, Gs2 forcontrolling the turning on and off of the bidirectional switch elementsS1, S2.

Specifically, the control circuit 200 generates the control signals G1,G2 and the control signals Gs1, Gs2 as described below. An AC outputvoltage command generation circuit 201 generates an AC output voltagecommand Vus* on the basis of the voltage Vrs of the AC power source 1.For instance, the AC output voltage command Vus* is synchronized withthe voltage Vrs of the AC power source 1, and has an amplitude identicalto that of the voltage rating of the AC power source 1.

The AC output voltage command Vus* can be set to an AC output voltagecommand that is asynchronous with the voltage Vrs of the AC power source1. The AC output voltage command Vus* can be set to an AC output voltagecommand having an amplitude different from that of the voltage rating ofthe AC power source 1.

The voltage Vrs of the AC power source 1 and the AC output voltagecommand Vus* are inputted to a voltage determination circuit 202. Thevoltage determination circuit 202 outputs a domain signal δ to which thecorresponding switching period T belongs, using the voltage Vrs of theAC power source 1 and the AC output voltage command Vus*. The domainsignal δ is classified into domains 1 to 6.

FIG. 4 is a diagram for explaining the relationship between domaindetermination performed by the control circuit 200, a pulse widthcommand a, and element selection. When the relationship between the ACoutput voltage command Vus* and the voltage Vrs of the AC power source 1is Vus*≧0 and Vrs<0, the voltage determination circuit 202 determinesthe switching period T to be domain 1.

When the relationship between the AC output voltage command Vus* and thevoltage Vrs of the AC power source 1 is Vus*≧0 and Vrs≧0 and Vrs<Vus*,the voltage determination circuit 202 determines the switching period Tto be domain 2.

When the relationship between the AC output voltage command Vus* and thevoltage Vrs of the AC power source 1 is Vus*≧0 and Vrs≧0 and Vrs≧Vus*,the voltage determination circuit 202 determines the switching period Tto be domain 3.

When the relationship between the AC output voltage command Vus* and thevoltage Vrs of the AC power source 1 is Vus*<0 and Vrs<0 and Vrs≦Vus*,the voltage determination circuit 202 determines the switching period Tto be domain 4.

When the relationship between the AC output voltage command Vus* and thevoltage Vrs of the AC power source 1 is Vus*<0 and Vrs<0 and Vrs>Vus*,the voltage determination circuit 202 determines the switching period Tto be domain 5.

When the relationship between the AC output voltage command Vus* and thevoltage Vrs of the AC power source 1 is Vus*<0 and Vrs≧0, the voltagedetermination circuit 202 determines the switching period T to be domain6. In each domain, one element from among the four elements is selectedas an H-arm element and another element is selected as an L-arm element.The remaining two elements not having been selected as an H-arm elementor an L-arm element constitute off-state arm elements.

The H-arm element is an element that by being turned on can output, tothe AC output terminal U, a voltage (first voltage), from among the fourlevels of voltage, that is equal to or higher than the AC output voltagecommand Vus* and that is closest to the AC output voltage command Vus*.The H-arm element is on during a time (H-arm on-state time)corresponding to the below-described pulse width command a.

The L-arm element is an element that by being turned on can output, tothe AC output terminal U, a voltage (second voltage), from among thefour levels of voltage, that is smaller than the AC output voltagecommand Vus* and that is closest to the AC output voltage command Vus*.The L-arm element is on during a time (L-arm on-state time) resultingfrom subtracting the H-arm on-state time from the switching period T.

Within the switching period T, the off-state arm elements are constantlyoff. Returning to FIG. 3, the pulse width command selection circuit 203receives the voltage Vrs of the AC power source 1, the voltage Vps ofthe DC power source Psp, the voltage Vns of the DC power source Psn, theAC output voltage command Vus* and the domain signal δ. On the basis ofthese input signals, the pulse width command selection circuit 203computes the pulse width command α (ratio of on-state time with respectto the switching period T) for the H-arm element.

The pulse width command α in domains 1 to 6 is worked out based on thefollowing expressions.

Pulse width command α in domain 1 α=Vus*/Vps  (1)

Pulse width command α in domain 2 α=(Vus*−Vrs)/(Vps−Vrs)  (2)

Pulse width command α in domain 3 α=Vus*/Vrs  (3)

Pulse width command α in domain 4 α=Vus*/Vrs  (4)

Pulse width command α in domain 5 α=(Vus*−Vrs)/(Vns−Vrs)  (5)

Pulse width command α in domain 6 α=Vus*/Vns  (6)

The comparator 204 receives the pulse width command α and a carriersignal Sc generated by a carrier signal generation circuit 206. Thecomparator 204 compares the pulse width command α and the carrier signalSc, and generates a signal Hon for turning on the H-arm element. Whenthe H-arm on-state signal Hon is at a high level, the H-arm element ison for the H-arm on-state time, within the switching period T.

A logic inverter 207 inverts the high level or low level of the H-armon-state signal Hon to the low level or the high level, and generates asignal Lon for turning on the L-arm element. When the L-arm on-statesignal Lon is at a high level, the L-arm element is on for the L-armon-state time, within the switching period T.

A pulse distribution circuit 205 receives the H-arm on-state signal Hon,the L-arm on-state signal Lon, and the domain signal δ. The pulsedistribution circuit 205 distributes the H-arm on-state signal Hon tothe control signal of the H-arm element that is selected in accordancewith the domain signal δ. The pulse distribution circuit 205 distributesthe L-arm on-state signal Lon to the control signal of the L-arm elementthat is selected in accordance with the domain signal δ. The pulsedistribution circuit 205 generates then control signals for turning offthe off-state arm elements for the duration of the switching period T.

As described above, the H-arm element is an element that, by beingturned on, can output, between the AC output terminals U and V, avoltage (first voltage), from among the four levels of voltage, that isequal to or higher than the AC output voltage command Vus* and that isclosest to the AC output voltage command Vus*. The L-arm element is anelement that, by being turned on, can output, between the AC outputterminals U and V, a voltage (second voltage), from among the fourlevels of voltage, that is smaller than the AC output voltage commandVus* and that is closest to the AC output voltage command Vus*.

In domain 1, with reference to FIG. 4, the H-arm element is theswitching element Q1, the L-arm element is the bidirectional switchelement S2, and the off-state arm elements are the switching element Q2and the bidirectional switch element S1. In domain 2, the H-arm elementis the switching element Q1, the L-arm element is the bidirectionalswitch element S1, and the off-state arm elements are the switchingelement Q2 and the bidirectional switch element S2. In domain 3, theH-arm element is the bidirectional switch element S1, the L-arm elementis the bidirectional switch element S2 and the off-state arm elementsare the switching elements Q1 and Q2. In domain 4, the H-arm element isthe bidirectional switch element S1, the L-arm element is thebidirectional switch element S2 and the off-state arm elements are theswitching elements Q1 and Q2. In domain 5, the H-arm element is theswitching element Q2, the L-arm element is the bidirectional switchelement S1, and the off-state arm elements are the switching element Q1and the bidirectional switch element S2. In domain 6, the H-arm elementis the switching element Q2, the L-arm element is the bidirectionalswitch element S2, and the off-state arm elements are the switchingelement Q1 and the bidirectional switch element S1.

An explanation follows next, with reference to FIG. 5A to FIG. 7E, onthe relationship between the AC output voltage Vus and the turning onand off operations of the four elements within the switching period T ina case where the AC output voltage command Vus* is positive (domains 1to 3).

FIG. 5A is a diagram for explaining a positive AC output voltage Vusoutputted between the AC output terminals U and V in domain 1. FIG. 5Bto FIG. 5E are diagrams for explaining the operations of the switchingelements Q1, Q2 and the bidirectional switch elements S1, S2 in thisdomain.

In this domain, the H-arm element is the switching element Q1. The L-armelement is the bidirectional switch element S2. The off-state armelements are the switching element Q2 and the bidirectional switchelement S1. Accordingly, the switching element Q1 is on for the on-timeT_(H1) (FIG. 5B). Therefore, the bidirectional switch element S2 is onfor the on-time T_(L1) (FIG. 5E). Herein, the switching element Q2 andthe bidirectional switch element S1 are off (FIG. 5C), FIG. 5D).

The on-time T_(H1) is a time calculated for the switching period T, onthe basis of the pulse width command α worked out based on Expression(1). The on-time T_(L), is a time resulting from subtracting the on-timeT_(H1) from the switching period T.

When the switching element Q1 is on, the voltage Vps of the DC powersource Psp, being the first voltage, is outputted between the AC outputterminals U and V. When the bidirectional switch element S2 is on, thezero voltage, being the second voltage, is outputted between the ACoutput terminals U and V (FIG. 5A). The average value of the voltagethat is outputted between the AC output terminals U and V is equal tothe AC output voltage command Vus*.

The voltages that are outputted within the switching period T may be thesecond voltage and the first voltage, in this order. The same applies tothe explanation hereafter. FIG. 6A is a diagram for explaining apositive AC output voltage Vus outputted between the AC output terminalsU and V in domain 2. FIG. 6B to FIG. 6E are diagrams for explaining theoperations of the switching elements Q1, Q2 and the bidirectional switchelements S1, S2 in this domain.

In this domain, the H-arm element is the switching element Q1. The L-armelement is the bidirectional switch element S1. The off-state armelements are the switching element Q2 and the bidirectional switchelement S2. Accordingly, the switching element Q1 is on for the on-timeT_(H2) (FIG. 6B). Thereafter, the bidirectional switch element S1 is onfor the on-time T_(L2) (FIG. 6D). The switching element Q2 and thebidirectional switch element S2 are off (FIG. 6C), FIG. 6E).

The on-time T_(H2) is a time calculated for the switching period T, onthe basis of the pulse width command α worked out based on Expression(2). The on-time T_(L2) is a time resulting from subtracting the on-timeT_(H2) from the switching period T.

When the switching element Q1 is on, the voltage Vps of the DC powersource Psp, being the first voltage, is outputted between the AC outputterminals U and V. When the bidirectional switch element S1 is on, thevoltage Vrs of the AC power source 1, being the second voltage, isoutputted between the AC output terminals U and V (FIG. 6A). The averagevalue of the voltage that is outputted between the AC output terminals Uand V is equal to the AC output voltage command Vus*.

FIG. 7A is a diagram for explaining a positive AC output voltage Vusoutputted between the AC output terminals U and V in domain 3. FIG. 7Bto FIG. 7E are diagrams for explaining the operations of the switchingelements Q1, Q2 and the bidirectional switch elements S1, S2 in thisdomain.

In this domain, the H-arm element is the bidirectional switch elementS1. The L-arm element is the bidirectional switch element S2. Theoff-state arm elements are the switching element Q1 and the switchingelement Q2. Accordingly, the bidirectional switch element S1 is on forthe on-time T_(H3) (FIG. 7B). Thereafter, the bidirectional switchelement S2 is on for the on-time T_(L3) (FIG. 7D). The switching elementQ1 and the switching element Q2 are off (FIG. 7C), FIG. 7E).

The on time T_(H3) is a time calculated for the switching period T, onthe basis of the pulse width command α worked out based on Expression(3). The on-time T_(L3) is a time resulting from subtracting the on-timeT_(H3) from the switching period T.

When the bidirectional switch element S1 is on, the voltage Vrs of theAC power source 1, being the first voltage, is outputted between the ACoutput terminals U and V. When the bidirectional switch element S2 ison, the zero voltage, being the second voltage, is outputted between theAC output terminals U and V (FIG. 7A). The average value of the voltagethat is outputted between the AC output terminals U and V is equal tothe AC output voltage command Vus*.

FIG. 8A to FIG. 10E are diagrams for explaining the relationship betweenthe AC output voltage Vus and the turning on and off of the fourelements within the switching period T in a case where the AC outputvoltage command Vus* is negative (domains 4 to 6).

FIGS. 8A-8E are diagrams for explaining the AC output voltage in domain4. Domain 4 is a domain wherein, by virtue of circuit symmetry, theswitching elements Q1, Q2 and the bidirectional switch elements S1, S2operate substantially in the same way as in the case of domain 3. Inthis domain, a negative voltage of which average voltage is equal to theAC output voltage command Vus* is outputted between the AC outputterminals U and V.

FIGS. 9A-9E are diagrams for explaining the AC output voltage in domain5. Domain 5 is a domain wherein, by virtue of circuit symmetry, theoperations of the switching element Q1 and the switching element Q2 arereversed, and operations substantially identical to those in the case ofdomain 2 are performed. In this domain, a negative voltage of whichaverage voltage is equal to the AC output voltage command Vus* isoutputted between the AC output terminals U and V.

FIGS. 10A-10E are diagrams for explaining the AC output voltage indomain 6. Domain 6 is a domain wherein, by virtue of circuit symmetry,the operations of the switching element Q1 and the switching element Q2are reversed, and operations substantially identical to those in thecase of domain 1 are performed. In this domain, a negative voltage ofwhich average voltage is equal to the AC output voltage command Vus* isoutputted between the AC output terminals U and V.

As described above, the power conversion circuit according to thepresent embodiment selects the H-arm element and L-arm element that arenecessary in order to generate the AC output voltage Vus that is equalto the AC output voltage command Vus*, at each switching period T. Thepower conversion circuit according to the present embodiment turns on,for a respective predefined time, the H-arm element and the L-armelement, within the switching period T; thereby, voltage such that theaverage voltage thereof is equal to the AC output voltage command Vus*can be generated between the AC output terminals U and V.

For instance, when the voltage Vrs of the AC power source 1 is lowerthan the AC output voltage command Vus*, the power conversion circuitaccording to the present embodiment superimposes, for a predefined time,the voltage Vps and Vns of the DC power source series circuit 30 on thevoltage Vrs of the AC power source 1, through operation in domain 2 anddomain 5, so that the AC output voltage Vus can be generated thereby.

When the voltage Vrs of the AC power source 1 is higher than the ACoutput voltage command Vus*, the power conversion circuit according tothe present embodiment steps down the voltage of the AC power source 1,through operation in domain 3 and domain 4, so that the AC outputvoltage Vus can be generated thereby.

The power conversion circuit according to the present embodiment canfurther generate AC output voltage Vus of opposite polarity to that ofthe voltage Vrs of the AC power source 1, through operation in domain 1and domain 6. The phase of the AC output voltage Vus that is generatedherein deviates significantly from that of the voltage Vrs of the ACpower source 1.

That is, the power converter according to the present embodiment canmaintain the AC output voltage Vus that is supplied to the load 6 at theAC output voltage command Vus*, by using the voltage Vrs of the AC powersource 1 and the voltages Vps, Vns of the DC power source series circuit30.

The power converter according to the present embodiment cannot outputvoltage that is higher than the voltage Vps of the DC power source Pspor voltage that is lower than the voltage Vns of the DC power sourcePsn. Accordingly, it is appropriate to perform a protective operation,which involves for instance turning off all elements, when the AC outputvoltage command Vus* is higher than the voltage Vps of the DC powersource Psp, or when the AC output voltage command Vus* is lower than thevoltage Vns of the DC power source Psn.

The switching element Q1 may be maintained constantly in an on-statewhen the AC output voltage command Vus* is higher than the voltage Vpsof the DC power source Psp. The switching element Q2 may be maintainedconstantly in an on-state when the AC output voltage command Vus* islower than the voltage Vns of the DC power source Psn.

In the power converter illustrated in FIG. 13, the turning on and offoperations of the switching elements of the inverter circuit areperformed between the positive-side voltage and the negative-sidevoltage of the DC power source series circuit. In the power converteraccording to the present embodiment, however, the turning on and offoperations of the switching elements and bidirectional switch elementsare performed between the first voltage and the second voltage. Asdescribed above, the first voltage is a voltage equal to or higher thanthe AC output voltage command Vus* and closest to the AC output voltagecommand Vus*. The second voltage is a voltage that is lower than the ACoutput voltage command Vus* and closest to the AC output voltage commandVus*. As FIG. 5A to FIG. 10E make clear, the voltage difference betweenthe first voltage and the second voltage is significantly smaller thanthe magnitudes of the voltages Vps, Vns of the DC power source.

Therefore, the switching losses that occur upon turning on and turningoff of the switching elements of the inverter circuit 4 according to thepresent embodiment are smaller than the switching losses of theswitching elements of the power converter illustrated in FIG. 13.Similarly, the switching losses that occur upon turning on and turningoff of the bidirectional switch elements of the bidirectional switchcircuit 100 according to the present embodiment are smaller than theswitching losses of the switching elements of the power converterillustrated in FIG. 13.

That is, the switching loss in the power converter according to thepresent embodiment can be made smaller than that of the inverter circuit4 illustrated in FIG. 13 when the switching frequency of the powerconverter according to the present embodiment is set to the sameswitching frequency as that of the inverter circuit 4 illustrated inFIG. 13.

Preferably, in particular, the AC output voltage Vus is synchronizedwith the voltage Vrs of the AC power source 1. Synchronizing the ACoutput voltage Vus to the voltage Vrs of the AC power source 1 makes itpossible to further reduce the voltage that is applied to the switchingelements Q1, Q2 and the bidirectional switch elements S1, S2. Theswitching loss incurred by these elements can be further reduced as aresult.

The AC output voltage Vus of the power converter according to thepresent embodiment varies between the first voltage and the secondvoltage. The voltage applied to the reactor Lf1 is accordingly smaller.Ripple current flowing in the reactor Lf1 is proportional to the voltagetime product (voltage variation range×voltage pulse width), andinversely proportional to the inductance value. For a same inductancevalue, the voltage time product is smaller in the power converter of thepresent embodiment, and the ripple current flowing in the reactor Lf1 isaccordingly smaller. A smaller ripple current entails a smaller loss(mainly iron loss) in the reactor Lf1, and it becomes therefore possibleto reduce loss in the reactor Lf1.

The inductance value of the reactor Lf1 can be reduced when ripplecurrents are set to be identical. The size of the reactor Lf1 can bereduced in such a case. Even upon occurrence of power failure in the ACpower source 1, the H-arm element and the L-arm element in the powerconverter of the present invention can be selected at each switchingperiod T, in accordance with the same logical process as when the ACpower source 1 is in a normal condition. The AC output voltage Vus canbe maintained at the AC output voltage command Vus* through turning onand off of the selected H-arm element and L-arm element in the same wayas when the AC power source 1 is in a normal condition.

In the power converter according to this embodiment, control formaintaining the AC output voltage Vus at the AC output voltage commandVus* requires therefore no means for detecting a power failure in the ACpower source 1.

FIG. 11 is a diagram for explaining another relationship between domaindetermination performed by the control circuit 200, the pulse widthcommand a, and element selection. FIGS. 12A-12E is a diagram forexplaining the AC output voltage Vus in domain 7, and the operations ofthe switching elements Q1, Q2 and the bidirectional switch elements S1,S2.

The configuration of the control circuit 200 is identical to that of thecontrol circuit illustrated in FIG. 3. Herein, however, the voltagedetermination circuit 202 determines also domain 7 in addition domains 1to 6 illustrated in FIG. 4. Domain 7 is a domain for outputting voltageof the AC power source 1 between the AC output terminals U and V.

The explanation thereafter with reference to FIG. 3, FIG. 11 and FIGS.12A-12E will focus on the operation of the control circuit 200 fordomain 7, and portions shared with domains 1 to 6 explained above willbe omitted as appropriate.

The AC output voltage command Vus* and the voltage Vrs of the AC powersource 1 are inputted, at each switching period T, to the voltagedetermination circuit 202. The voltage determination circuit 202determines the switching period T to be domain 7 when the relationshipbetween the two voltages satisfies the condition |Vus*−Vrs|<ΔVus*. Thevoltage determination circuit 202 outputs then a domain signal δdenoting domain 7.

Herein, ΔVus* is a reference quantity for determining that the value ofthe voltage Vrs of the AC power source 1 lies within a predefined rangewith respect to the value of the AC output voltage command Vus*. In acase where the load 6 allows for a fluctuation of the input voltagewithin a range of AC output voltage command Vus*±10%, the referencequantity ΔVus* is equivalent to 10% of the AC output voltage commandVus*. The reference quantity ΔVus* may be established in accordance withsome other condition.

Upon input of a domain signal δ denoting domain 7, the pulse widthcommand selection circuit 203 fixes the pulse width command α to 1.0.When the pulse width command α is 1.0, the comparator 204 generates thesignal Hon for turning on the H-arm element throughout the switchingperiod T, regardless of the magnitude of the carrier signal Sc. In theswitching period T, thus, the H-arm on-state signal Hon is constantly ata high level and the L-arm on-state signal Lon is constantly at a lowlevel.

Upon input of the domain signal δ denoting domain 7, the pulsedistribution circuit 205 sets the bidirectional switch element S1 as theH-arm element. The pulse distribution circuit 205 sets the switchingelements Q1, Q2 and the bidirectional switch element S2 as the off-statearm elements. Accordingly, the pulse distribution circuit 205 outputs acontrol signal Gs1 of the bidirectional switch element S1 that is at ahigh level during the switching period T, and control signals G1, G2 andGs2 of the switching elements Q1, Q2 and the bidirectional switchelement S2 that are at a low level during the switching period T.

Therefore, the bidirectional switch element S1 is on and the switchingelements Q1, Q2 and the bidirectional switch element S2 are off duringthe switching period T determined to be domain 7. The voltage Vrs of theAC power source 1 is outputted thus between the AC output terminals Uand V through the operation of the four elements.

Also in the case where the AC output voltage command Vus* has negativepolarity, the control circuit 200 performs the same operations as in thecase where the AC output voltage command Vus* has positive polarity. Inthe switching period T determined to be domain 7, only the bidirectionalswitch element S1 is on, while the switching elements Q1, Q2 and thebidirectional switch element S2 are off. Accordingly, only thebidirectional switch element S1 incurs conduction loss on account ofcurrent application. No conduction loss occurs in the switching elementsQ1, Q2 and the bidirectional switch element S2, since no current flowsthrough these. Moreover, there occurs no switching loss, since none ofthe elements undergoes an on or off operation.

Therefore, power loss can be further reduced by providing domain 7 inthe operation of the power conversion circuit. Next, FIG. 13 is adiagram for explaining a second embodiment of the power converter of thepresent invention. The power converter according to the presentembodiment is configured by using herein a half-bridge converter circuit31 as the DC power source series circuit 30 of the embodimentillustrated in FIG. 1.

The converter circuit 31 has, as main constituent elements, a seriescircuit of a positive-side switching element Qp and a negative-sideswitching element Qn, a series circuit of a positive-side capacitor Cpand a negative-side capacitor Cn, and a reactor L. One end of thereactor L is connected to one end of the AC power source 1, and theother end is connected to a connection midpoint of the switchingelements Qp, Qn. The series circuit of the capacitors Cp, Cn isconnected in parallel to the series circuit of the switching elementsQp, Qn. The connection midpoint of the capacitors Cp, Cn is connected tothe other end of the AC power source 1, and is also connected to the ACoutput terminal V.

When the voltage of the AC power source 1 has positive polarity withrespect to the AC output terminal V, firstly, the switching element Qnis turned on, and the switching element Qp is turned off. Throughturning-on of the switching element Qn, voltage resulting from addingthe voltage of the capacitor Cn and the voltage of the AC power source 1is applied to the reactor L, and energy accumulates in the reactor L.Next, the switching element Qn is turned off and the switching elementQp is turned on. When the switching element Qn is turned off, the energyaccumulated in the reactor L is charged to the capacitor Cp.

When the voltage of the AC power source 1 has negative polarity withrespect to the AC output terminal V, firstly, the switching element Qpis turned on, and the switching element Qn is turned off. Throughturning-on of the switching element Qp, a voltage resulting from addingthe voltage of the capacitor Cp and the voltage of the AC power source 1is applied to the reactor L, and energy accumulates in the reactor L.Next, the switching element Qp is turned off, and the switching elementQn is turned on. When the switching element Qp is turned off, the energyaccumulated in the reactor L is charged to the capacitor Cn.

The on-off operations of the switching elements Qp, Qn are performed ata frequency that is sufficiently higher than the frequency of the ACpower source 1. Through the on-off operations of the switching elementsQp, Qn, the voltage of the capacitor Cp and the capacitor Cn ismaintained at a predefined voltage that is higher than the voltage ofthe AC power source 1.

The DC power source series circuit 30 of the power converter of thepresent invention can be configured thus in the form of a half-bridgeconverter 31. The capacitor Cp of the half-bridge converter 31corresponds to the positive-side DC power source Psp of the DC powersource series circuit 30. The capacitor Cn of the half-bridge converter31 corresponds to the negative-side DC power source Psn of the DC powersource series circuit 30.

In the present embodiment, the action and effect of circuits other thanthe half-bridge converter 31 are identical to the action and effect ofthe power converter according to the first embodiment explained withreference to FIG. 1 through FIG. 12E.

Next, FIG. 14 is a diagram for explaining a third embodiment of thepower converter of the present invention. In the power converteraccording to this embodiment, the DC power source series circuit 30 ofthe embodiment illustrated in FIG. 1 is configured in the form of athree-level rectifier 32.

The three-level rectifier 32 has, as main constituent elements, a seriescircuit of a positive-side diode Dp and a negative-side diode Dn, theseries circuit of the positive-side capacitor Cp and the negative-sidecapacitor Cn, a bidirectional switch element S3, and the reactor L. Oneend of the reactor L is connected to one end of the AC power source 1,and the other end of the reactor L is connected to a connection midpointof the diodes Dp, Dn. The series circuit of the capacitors Cp, Cn isconnected in parallel to the series circuit of the diodes Dp, Dn. Theconnection midpoint of the capacitors Cp, Cn is connected to the otherend of the AC power source 1, and is also connected to the AC outputterminal V. The bidirectional switch element S3 is connected between theconnection midpoint of the diodes Dp, Dn and the connection midpoint ofthe capacitors Cp, Cn.

When the voltage of the AC power source 1 has positive polarity withrespect to the AC output terminal V, firstly the bidirectional switchelement S3 is turned on. Upon turning-on of the bidirectional switchelement S3, the voltage of the AC power source 1 is applied to thereactor L, and energy accumulates in the reactor L. Next, thebidirectional switch element S3 is turned off. Upon turning off of thebidirectional switch element S3, the energy accumulated in the reactor Lis charged to the capacitor Cp.

On the other hand, when the voltage of the AC power source 1 hasnegative polarity with respect to the AC output terminal V, firstly thebidirectional switch element S3 is turned on. Upon turning-on of thebidirectional switch element S3, the voltage of the AC power source 1 isapplied to the reactor L, and energy accumulates in the reactor L. Next,the bidirectional switch element S3 is turned off. Upon turning off ofthe bidirectional switch element S3, the energy accumulated in thereactor L is charged to the capacitor Cn.

The on-off operations of the bidirectional switch element S3 areperformed at a frequency that is sufficiently higher than the frequencyof the AC power source 1. Through the on-off operations of bidirectionalswitch element S3, the voltage of the capacitor Cp and the capacitor Cnis maintained at a predefined voltage that is higher than the voltage ofthe AC power source 1.

The DC power source series circuit 30 of the power converter of thepresent invention can be configured thus in the form of the three-levelrectifier 32. The capacitor Cp of the three-level rectifier 32corresponds to the positive-side DC power source Psp of the DC powersource series circuit 30. The capacitor Cn of the three-level rectifier32 corresponds to the negative-side DC power source Psn of the DC powersource series circuit 30.

In the present embodiment, the action and effect of circuits other thanthe three-level rectifier 32 are identical to the action and effect ofthe power converter according to the first embodiment explained withreference to FIG. 1 through FIG. 12E.

1. A power converter that outputs AC voltage based on an AC outputvoltage command, comprising: a single-phase AC power source having afirst AC terminal and a second AC terminal; and a DC power source seriescircuit, resulting from connecting in series a first DC power source anda second DC power source such that a neutral terminal, which is aconnection point of the first DC power source and the second DC powersource, is connected to the second AC terminal, wherein the powerconverter outputs voltage of any one level selected from among fourlevels of voltage of a positive-side voltage of the DC power sourceseries circuit, a negative-side voltage of the DC power source seriescircuit, zero voltage being the potential of the neutral terminal, and avoltage of the AC power source.
 2. The power converter according toclaim 1, wherein the positive-side voltage of the DC power source seriescircuit and the negative-side voltage of the DC power source seriescircuit are larger than an amplitude value of the voltage of the ACpower source.
 3. The power converter according to claim 2, wherein ateach control period that is continuous and has a pre-established timewidth, the power converter sets, as a first voltage, a voltage of onelevel selected from among the four levels of voltage, and sets, as asecond voltage, a voltage of another level selected from among the fourlevels of voltage, and outputs AC output voltage corresponding to the ACoutput voltage command by complementarily outputting the first voltageand the second voltage over respective predefined time widths.
 4. Thepower converter according to claim 3, wherein the first voltage and thesecond voltage are voltages selected on the basis of the AC outputvoltage command and the voltage of the AC power source.
 5. The powerconverter according to claim 2, wherein at each control period that iscontinuous and has a pre-established time width, the power converterselects as a first voltage, from among the four levels of voltage, avoltage having an absolute value equal to or higher than an absolutevalue of the AC output voltage command and having a value that isclosest to a value of the AC output voltage command, and selects as asecond voltage, from among the four levels of voltage, a voltage havingan absolute value smaller than the absolute value of the AC outputvoltage command, and having a value that is closest to the value of theAC output voltage command; and sets a time ratio of an output time ofthe first voltage and an output time of the second voltage to apredefined value within the control period, and complementarily outputsthe first voltage and the second voltage, to thereby output AC voltagecorresponding to the AC output voltage command.
 6. The power converteraccording to claim 2, wherein at each control period that is continuousand has a pre-established time width, the power converter sets thepositive-side voltage of the DC power source series circuit as a firstvoltage and sets the zero voltage as a second voltage when the AC outputvoltage command is equal to or higher than the zero voltage and thevoltage of the AC power source is lower than the zero voltage, sets thepositive-side voltage of the DC power source series circuit as the firstvoltage and sets the voltage of the AC power source as the secondvoltage when the AC output voltage command is equal to or higher thanthe zero voltage, the voltage of the AC power source is equal to orhigher than the zero voltage, and the AC output voltage command ishigher than the voltage of the AC power source, sets the voltage of theAC power source as the first voltage and sets the zero voltage as thesecond voltage when the AC output voltage command is equal to or higherthan the zero voltage, the voltage of the AC power source is equal to orhigher than the zero voltage and the AC output voltage command is equalto or lower than the voltage of the AC power source, sets the voltage ofthe AC power source as the first voltage and sets the zero voltage asthe second voltage when the AC output voltage command is lower than thezero voltage, the voltage of the AC power source is lower than the zerovoltage and the AC output voltage command is equal to or higher than thevoltage of the AC power source, sets the negative-side voltage of the DCpower source series circuit as the first voltage and sets the voltage ofthe AC power source as the second voltage when the AC output voltagecommand is lower than the zero voltage, the voltage of the AC powersource is lower than the zero voltage and the AC output voltage commandis lower than the voltage of the AC power source, sets the negative-sidevoltage of the DC power source series circuit as the first voltage andsets the zero voltage as the second voltage when the AC output voltagecommand is lower than the zero voltage and the voltage of the AC powersource is equal to or higher than the zero voltage, and sets a timeratio of an output time of the first voltage and an output time of thesecond voltage to a predefined value within the control period, andcomplementarily outputs the first voltage and the second voltage, tothereby output an AC voltage corresponding to the AC output voltagecommand.
 7. The power converter according to claim 5, wherein the outputtime of the first voltage is established on the basis of the AC outputvoltage command, the first voltage and the second voltage, and theoutput time of the second voltage is a time resulting from subtractingthe output time of the first voltage from the time of each controlperiod.
 8. The power converter according to claim 5, wherein the outputtime of the first voltage is a time corresponding to a value obtained bydividing a difference voltage between the AC output voltage and thesecond voltage by a difference voltage between the first voltage and thesecond voltage.
 9. The power converter according to claim 3, wherein anaverage value of the AC output voltage in the control period is equal toan average value of the AC output voltage command within that controlperiod.
 10. The power converter according to claim 3, wherein the ACoutput voltage command is synchronized with the voltage of the AC powersource.
 11. The power converter according to claim 10, wherein the powerconverter outputs the voltage of the AC power source when a deviationbetween the voltage of the AC power source and the AC output voltagecommand lies within a pre-established range.
 12. A power converter thatoutputs AC voltage based on an AC output voltage command, comprising: asingle-phase AC power source having a first AC terminal and a second ACterminal; a DC power source series circuit, resulting from connecting inseries a first DC power source and a second DC power source such that aneutral terminal, which is a connection point of the first DC powersource and the second DC power source, is connected to the second ACterminal; a switching element series circuit resulting from connectingin series a positive-side switching element that is connected to apositive-side terminal of the DC power source series circuit, and anegative-side switching element that is connected to a negative-sideterminal of the DC power source series circuit; a first AC outputterminal connected to a connection point of the positive-side switchingelement and the negative-side switching element; a second AC outputterminal connected to the neutral terminal; a first bidirectional switchelement one end of which is connected to the first AC output terminaland the other end of which is connected to a first AC terminal of the ACpower source; and a second bidirectional switch element one end of whichis connected to the first AC output terminal and the other end of whichis connected to the neutral terminal of the DC power source seriescircuit, wherein the power converter outputs, to the first AC outputterminal, voltage of any one level selected from among four levels ofvoltage of a positive-side voltage of the DC power source seriescircuit, a negative-side voltage of the DC power source series circuit,zero voltage being the potential of the neutral terminal, and a voltageof the AC power source.
 13. The power converter according to claim 12,wherein the positive-side voltage of the DC power source series circuitand the negative-side voltage of the DC power source series circuit arelarger than an amplitude value of the voltage of the AC power source.14. The power converter according to claim 13, wherein at each controlperiod that is continuous and has a pre-established time width, thepower converter sets, as a first voltage, a voltage of one levelselected from among the four levels of voltage, and sets, as a secondvoltage, a voltage of another level selected from among the four levelsof voltage, and complementarily outputs the first voltage and the secondvoltage to the first AC output terminal, over respective predefined timewidths.
 15. The power converter according to claim 14, wherein the firstvoltage and the second voltage are voltages selected on the basis of theAC output voltage command and the voltage of the AC power source. 16.The power converter according to claim 13, wherein at each controlperiod that is continuous and has a pre-established time width, thepower converter selects as a first voltage, from among the four levelsof voltage, a voltage having an absolute value equal to or higher thanan absolute value of the AC output voltage command and having a valuethat is closest to a value of the AC output voltage command, and selectsas a second voltage, from among the four levels of voltage, a voltagehaving an absolute value smaller than the absolute value of the ACoutput voltage command, and having a value that is closest to the valueof the AC output voltage command, sets a time ratio of an output time ofthe first voltage and an output time of the second voltage to apredefined value within the control period, and complementarily outputsthe first voltage and the second voltage to the first AC outputterminal.
 17. The power converter according to claim 13, wherein at eachcontrol period that is continuous and has a pre-established time width,the power converter sets a positive voltage of the DC power sourceseries circuit as a first voltage and sets the zero voltage as a secondvoltage when the AC output voltage command is equal to or higher thanthe zero voltage and the voltage of the AC power source is lower thanthe zero voltage, sets the positive voltage of the DC power sourceseries circuit as the first voltage and sets the voltage of the AC powersource as the second voltage when the AC output voltage command is equalto or higher than the zero voltage, the voltage of the AC power sourceis equal to or higher than the zero voltage, and the AC output voltagecommand is higher than the voltage of the AC power source, sets thevoltage of the AC power source as the first voltage and sets the zerovoltage as the second voltage when the AC output voltage command isequal to or higher than the zero voltage, the voltage of the AC powersource is equal to or higher than the zero voltage and the AC outputvoltage command is equal to or lower than the voltage of the AC powersource, sets the voltage of the AC power source as the first voltage andsets the zero voltage as the second voltage when the AC output voltagecommand is lower than the zero voltage, the voltage of the AC powersource is lower than the zero voltage and the AC output voltage commandis equal to or higher than the voltage of the AC power source, sets anegative voltage of the DC power source series circuit as the firstvoltage and sets the voltage of the AC power source as the secondvoltage when the AC output voltage command is lower than the zerovoltage, the voltage of the AC power source is lower than the zerovoltage and the AC output voltage command is lower than the voltage ofthe AC power source, sets the negative voltage of the DC power sourceseries circuit as the first voltage and sets the zero voltage as thesecond voltage when the AC output voltage command is lower than the zerovoltage and the voltage of the AC power source is equal to or higherthan the zero voltage, sets a time ratio of an output time of the firstvoltage and an output time of the second voltage to a predefined valuewithin the control period, and complementarily outputs the first voltageand the second voltage to the first AC output terminal.
 18. The powerconverter according to claim 16, wherein the output time of the firstvoltage is established on the basis of the AC output voltage command,the first voltage and the second voltage, and the output time of thesecond voltage is a time resulting from subtracting the output time ofthe first voltage from the time of each control period.
 19. The powerconverter according to claim 16, wherein the output time of the firstvoltage is a time corresponding to a value obtained by dividing adifference voltage between the AC output voltage and the second voltageby a difference voltage between the first voltage and the secondvoltage.
 20. The power converter according to claim 14, wherein anaverage value of the AC output voltage in the control period is equal toan average value of the AC output voltage command within that controlperiod.
 21. The power converter according to claim 14, wherein the ACoutput voltage command is synchronized with the voltage of the AC powersource.
 22. The power converter according to claim 21, wherein the powerconverter outputs, to the first AC output terminal, the voltage of theAC power source when a deviation between the voltage of the AC powersource and the AC output voltage command lies within a pre-establishedrange.